Hi Dyedinthewool. Now know for certain what that 'dust' is on the rear end feathers. NORTHERN FOWL MITE. Reason we know is because our pet Buff Orpington Cockerel has been a bit down recently and doing a lot of preening. Cockerels seem prone to it in preference to hens, probably because they have so much longer feathers. Very close inspection with a strong torch and magnifying glass revealed thousnads of tiny mite, about half a dozen to a pin head, crawling over scabs on the skin. They eat the skin to get to the meal of flesh underneath. Recognised the dusty feathers from your photograph. We've given him Frontline spray at 3 squirts per Kg body weight. His bed is full of them this morning -dead! Problem is they hide under the scabs, so he might need more treatment like a medicated shampoo and cream on the scabs.
It is difficult to spot and initially looks like a dirty bum. Of course he doesn't like us messing with his feathers so it wasn't until he was really run down we could have a good look, despite his squeals of pain. He seems better this morning but it can be fatal. Huge scabs on him though, over his entire bum and right round his hackles.
They get very dehydrated, which would explain why a squashy crop results, drinking a lot of water. Suprised the vet didn't spot it, but you went in for a crop problem.
We have several with 'pendulous crops' but only one has resulted in a fungal problem. It was so far advanced when we spotted it, so after a month of treatment and no improvement, even after force feeding we pts.